Professor Norman Sherry's battle to understand and explain the life of Graham Greene is a legend of modern biography. When they first met 30 years ago, Greene had read and admired one of Sherry's books - a study of Conrad - and Sherry, who was head of English at the University of Lancaster had, reportedly, read one of Greene's, A Gun for Sale. The biographer was looking for a new subject, and the author was tentatively encouraging.

After another meeting in London, Greene was knocked down by a taxi and Sherry made a joke about the incident. Greene liked jokes and it seems that, on the basis of this one, he gave Sherry the job coveted by writers who knew his work much better. He sent his newly appointed biographer a map of the world, with all the places he had travelled to marked in red, asked him not to interview certain women and promised never to lie to him - though he also made it clear that he would not always be helpful.

The first volume of biography, covering Greene's childhood, his early suicide attempts and conversion to Catholicism, was well-written and widely praised. The second volume, a patchier affair, appeared three years after Greene's death in 1991. It encompassed the years when Greene was creating his greatest characters, men John Mortimer has described as living "on the dangerous edge of faith". There was also Greene's love for a married American, Catherine Walston, which inspired The End of the Affair .

With this last volume, it seems that all the fight has gone out of Sherry. The book is badly written, full of lazy assumptions and statements of the crashingly obvious. Borrowing the title of a Greene novel, Sherry has apparently described himself as "a burnt-out case", attributing illness and family problems to his exhausting task.

His aim, as set out in the introduction, is to recreate Greene's "total experience in a particular area at a particular time". To that end Sherry spent seven years travelling the world, tracking down people Greene had known, and even contracting some of the same illnesses (he lost part of his intestine this way). One thinks of Pierre Menard, a character created by Borges, who wanted to think and see like Cervantes so that he could write Don Quixote anew.

Here, for example, is Sherry in Mexico, reporting back on the man who inspired the Judas figure in The Power and the Glory: "The mannerisms Greene noted, including the scratching under his armpit, were repeated in my company, but his way of life made this understandable. His mouth was toothless, the two fangs described by Greene gone, so that what Greene saw and felt, I did not."

Clearly the lack of those two fangs disappoints Sherry: the "total experience" he wants to give us is compromised. But even if Sherry had loved the same women, drunk the same whisky and suffered from the same "large, painful, internal pile" it would not bring us closer to Greene on the page. He cannot "see what Greene saw" but he ought to be able to do something more valuable and that is show us the man he claims to have known better "than I know the lines on my face". Sherry met Greene lots of times and interviewed him extensively. Yet, over the course of 800 pages, he fails to bring him to life.

During the years covered here, 1955 to 1991, Greene travelled to Haiti, the Congo and Cuba. He wrote novels including The Comedians, A Burnt-Out Case, Our Man in Havana, Monsignor Quixote and Travels with My Aunt, as well as plays and film-scripts. He championed the embattled regimes of Panama and Nicaragua, and finally lost hope of a life with Walston. His last 30 years were spent in Antibes, close to another married woman, Yvonne Cloetta.

Sherry plots all these events, but in his hands we discover little about the feeling or motivation behind them. Many questions remain. Was Greene able to love women only when deceit was part of the equation? Was religion a way to add drama to his work, and to his life? How should we reconcile the "hollow", self-hating Greene with the funny and affectionate man his friends remember? Sherry does not do justice to his subject's remarkable generosity - Greene supported, often anonymously, a number of struggling writers as well as a rag-tag bunch of hard-up friends. He makes simple things complicated and vice versa, tossing out that Greene suffered from manic depression, or voicing opinions he seems not to trust himself. Greene never loved anyone as much as Catherine, writes Sherry, adding "(I might be wrong)".

At other times he is simply defeated by the job of description. Greene was "special, unquestionably so", he tells us weakly, while Walston gets the luxury of italics: "Catherine was special."

It becomes a challenge to see what we can of Greene in spite of this intermediary. It is like jostling with someone to spy through a key-hole. You want to yank Sherry away, to get a better look at Greene, but his commentary obscures the view. Many of his observations mean next to nothing. "He is cornered, truth dripping slow," Sherry tells us sagely, and elsewhere: "[Greene] crossed the shadow-line, after casting a savage, sceptical look at his own face looking back from the mirror, over to a consuming vacancy reflected there, in an exhaustion as heavy as the sea."

The purple prose makes you long for Greene, whose voice remains compelling in letters, diary entries and articles. To Walston he writes: "I love you dearly & I hate myself so much - & it's the second half that seems to cause the trouble ... there's nothing in life one values like you - not work or children or books." And later: "We are nearly together again - put out your hand."

It is left to Greene himself to give the most coherent explanation of the moral ambivalence in his work and life: "[A writer] stands for the victims, and the victims change. Loyalty confines you to accepted opinions: loyalty forbids you to comprehend sympathetically your dissident fellows; but disloyalty encourages you to roam through any human mind: it gives the novelist an extra dimension of understanding."

There are other mercifully clear voices here too. Rebecca West is memorable on Greene's treatment of treachery. Evelyn Waugh and Anthony Burgess tackle his religious beliefs. Sherry piggybacks on the eloquence of others. But there is very little in the way of new commentary, possibly because Sherry has alienated the available sources or did not dare approach them. Louise Dennys, Greene's niece, editor and friend, was never asked for an interview.

After 30 years, and the toll that has been taken on his personal life, it would be understandable if Sherry no longer admired or even liked Greene (he hints that he loved Greene's siblings more). All the same, his subject's legacy is not served well by this final volume of biography. Greene, perhaps, must share some of the blame. The novelist, who was famously careless with his life, may have been careless with his Life, too.

It is terrible to read that, hours before he died, and just before falling into a final coma, Greene stopped taking morphia so that he could have a final word with his confessor, Fr Leopoldo Duran, and try to address some questions of Sherry's.

And what were the questions? He never even tells us.

· Miranda France is the author of Bad Times in Buenos Aires (1998) and Don Quixote's Delusions: Travels in Castilian Spain (2001).